The future VoIP (Voice over IP) networks are being designed using the layered approach, consisting of IM (IP Multimedia) networks providing the Application Level services over some Transport Level access network with maximum possible independence between the two layers. The 3GPP standardization efforts for R00 were focused on GPRS as the Transport Level access network.
In order to access the IP Multimedia services via the GPRS access network, the UE (UE refers to User Equipment or Application; whenever UE or User Equipment is referred to later in this document, it may also refer to possible User Agent or Application) must perform GPRS attach and activate the necessary PDP (Packet Data Protocol) context first, before registering with the IM network. During the PDP context activation, the GPRS network (GGSN (Gateway GPRS Support Node), more specifically) allocates to the UE an IP address, which is used by the GPRS network to address the UE. This IP address is conveyed to the IM network during the application level registration and is used also by the IM network to address the UE.
In most of the cases, the IP addresses are allocated to the user dynamically by the GPRS network during the PDP context activation. This can be arranged by two ways: either a GGSN has a pool of IP addresses or the DHCP (Dynamic Host Configuration Protocol) server allocates the address. An IP address thus allocated is freed when the PDP context is deactivated explicitly by the UE performing PDP context deactivation or implicitly, for example, after the UE becomes unreachable or fails to perform a periodic routing area update (RAU). The same IP address could be re-allocated to a different user by the GPRS network.
It may be possible that the UE explicitly performs PDP context deactivation with the GPRS network but does not perform any specific de-registration with the IM network. It is also possible that a PDP context associated with the UE is implicitly deactivated by the GPRS network for some reason as mentioned above. In all these situations, the IM network is not aware that the IP address of the UE has become invalid, or even worse, the IP address has been allocated to another user. In the later case, messages coming from the IM network (e.g., in call setup) might be delivered to a different UE which happens to use that IP address at present.